Introduction

In general, micro and nanoscale systems can be fabricated using either bottom-up or top-down approaches and strategies that have elements of both. Bottom-up approaches start from certain processes that result in a higher-ordered and -organized structure. Examples of bottom-up approaches include systems that self-assemble, a process that is triggered by a local change in a chemical or physical condition. Related techniques include templating and scaffolding methods, such as biomineralization, which rely on backbone structures to support and guide the nucleation and growth of a nanomaterial.

In a top-down approach complex structures are built up by patterning layers upon layers from the surface of a planar substrate. The realization of the single layer is based on the lithographic process, which in order to define the whole fabrication process can be roughly summarized in three steps: patterning, etching and depositing (See figure below).


The typical tools of a nanofabrication facility can be classified into two categories, those that precisely engineer matter (etching and depositing) and those that define the shape of the elements to be engineered (patterning, mainly lithographic techniques).

Lithography is the principal mechanism for pattern definition in micromachining.

Resist materials are usually organic polymers consisting of long molecular chains. When resist is exposed to a radiation source of a specific a wavelength, the chemical resistance of the resist to developer solution changes.

Depending on the prevalence of chain scission or chain bonding-in other  words, depolymerization (decrease in molecular weight) or  polymerization-(increase in molecular weight) of the irradiated zone (See figure) the resist is termed a "positive" or a "negative" resist.  


For positive resists, material will become more soluble in exposed regions, while a negative photoresist will become less soluble in exposed regions during development. The most important properties of a resist are sensitivity and contrast, which are directly linked to the resolution capability of the resist, directly influencing resist profiles and linewidth control.


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